1. Technical Field
This invention relates in general to electronic displays and, more particularly, to optimization of duty cycles for a display using solid state illumination.
2. Description of the Related Art
Spatial light modulation (SLM) display systems are visual display systems that are used as an alternative to conventional cathode-ray tube (CRT) systems. SLM systems are used in a variety of applications such as televisions and video projectors. One type of SLM may be referred to as a projection display system. Due to their superior clarity and performance, they are often used in high-end applications such as high-definition television (HDTV).
One popular projection display system is commercially available as DLP® (or Digital Light Processing®). DLP® utilizes a digital micromirror device (DMD), an array of thousands of tiny mirrors to properly reflect light from the light source to produce the image for display.
FIG. 1 illustrates a simplified configuration diagram illustrating selected components of an exemplary prior art display system 10. The display system 10 includes various components that define an optical path 5 between light source 11 and display screen 19. Light source 11 may be, for example, an ultra-high pressure (UHP) arc lamp. Display screen 19 may be separate from the display system 10 for a video projector or may be part of the display system 10 for a television. The display screen presents the visual image display intended to be seen by the viewer.
In operation, light emitted from the light source 11 is applied through a first condenser lens 12 and then through a rotating color wheel 13. Color wheel 13 will typically rotate at least once per frame (of the image to be displayed). The light passing through the color wheel 13 next passes through a second condenser lens 17 before illuminating DMD chip 15. It is chiefly the DMD chip 15 that modulates the light traveling through optical path 5 to produce a visual image.
To generate the images, the DMD chip 15 includes an array of tiny mirror elements, or micromirrors (typically on the order of one million of them). Each mirror element is separately controllable. For example, they may be mounted on a torsion hinge and support post above a memory cell of a CMOS static RAM as shown in FIG. 2. FIG. 2 shows a portion of a typical DMD chip 15 having mirror elements 21 suspended over a substrate 23. Electrostatic attraction between the mirror 21 and an address electrode 25 causes the mirror to twist or pivot, in either of two directions, about an axis formed by a pair of torsion beam hinges 27a and 27b. Typically, the mirror rotates about these hinges until the rotation is mechanically stopped. The movable micromirror tilts into the on or off states by electrostatic forces depending on the data written to the cell. The tilt of the mirror is on the order of plus 10 degrees (on) or minus 10 degrees (off) to modulate the light that is incident on the surface.
The DMD's are controlled by electronic circuitry (not shown) that has been fabricated on the silicon substrate 23 and is generally disposed under the DMD micromirror array. The circuitry includes an array of memory cells (also not shown), typically one memory cell for each DMD element, connected to the address electrodes 25. The output of a memory cell is connected to one of the two address electrodes and the inverted output of a memory cell is connected to the other address electrode.
The operation data is provided by a timing and control circuit 17 as determined from signal processing circuitry according to an image source 16 (as shown in FIG. 1). Once data is written to each memory cell in the array, a voltage is applied to the individual DMD mirrors 21 creating a large enough voltage differential between the mirrors 21 and the address electrodes 25 to cause the mirrors to rotate or tilt in the direction of the greatest voltage potential. Since the electrostatic attraction grows stronger as a mirror is rotated near an address electrode, the memory cell contents may be changed without altering the position of the mirrors once the mirrors are fully rotated. Thus, the memory cells may be loaded with new data while the array is displaying previous data.
As should be apparent, the rotation of the individual mirror elements 21 determines the amount and quality of light that will be directed at lens 18. The light reflected from any of the mirrors may pass through a projection lens 18 in order to create images on the screen 19. The intensity of each pixel displayed on the screen 19 is determined by the amount of time the mirror 21 corresponding to a particular pixel directs light toward screen 31. For example, each pixel may have 256 intensity levels for each color (e.g., red, green or blue). If the color level selected for a particular pixel at a particular time is 128, then the corresponding mirror would direct light toward that area of screen 31 for ½ (e.g., 128/256) of the frame time.
More recently, LEDs are used for the light source 11, rather than a lamp. LEDs provide significant advantages over white light lamps: (1) the LEDs have a longer expected life and (2) LEDs can have different associated colors, therefore red (R), blue (B) and green (G) LEDs can be used in order to eliminate the color wheel 13. By eliminating the color wheel, a moving part is eliminated, but, further, the control 17 does not need to account for changing colors sweeping across the mirrors as the segments of the color wheel rotate in from of the light.
FIG. 3 is a simplified configuration diagram illustrating selected components of an exemplary optical path 20 using LEDs. As with the example of FIG. 1, optical path 20 is part of a projection display system (although the projection lens and the display screen are not shown in FIG. 3). Exemplary optical path 20 of FIG. 3 is a “fixed array” system, having three stationary arrays; red array 28, green array 30, and blue array 32. No moving parts, such as color wheel 13 shown in FIG. 1, are needed. The light is applied sequentially by turning on and off each of the red, green, and blue arrays.
In operation, light from blue LED array 32 is transmitted via lens 33 through filter 34 and filter 35 to optical integrator 36. Likewise, light from green LED array 30 is passes through lens 31 and then is reflected from filter 34 but then transmitted through filter 35 to optical integrator 36. Light from red LED array 26 is reflected from filter 35 to optical integrator 36. Light from optical integrator 36 is transmitted to (and through) relay lenses 37 and 38, from where it is directed to DMD array 15. Light from DMD array 15 is then selectively directed to a projection lens (not shown) and on to a screen or other display medium (also not shown).
FIG. 4 illustrates a sequence of video frames 50, where each frame typically displayed a multiple sub-frames 51 of red, green and blue sub-sequences 52. The primary color sub-sequences are not necessarily the same length; as shown, the green sub-sequence is longer than the red or blue sub-sequences. The allocation of a frame between different light colors is referred to herein as the duty cycle. In the illustrated embodiment, the fractional allocation of the frame is approximately 0.25 R, 0.5 G and 0.25 B. While each color is shown in FIG. 4 as being a single sub-sequence, one or more of the colors may be split into multiple sub-sequences; for example, for the illustrated allocation of colors, a sequence of primary color sub-sequences may be: R, G, B, G, where each of the four sub-sequences is approximately 0.25 of the whole.
Other details regarding the operation of a digital display system are provided in U.S. Pub. Nos. US2005/0146541 and US2005/0068464, which are incorporated by reference herein.
Another advantage of LED technology is that the duty cycle for a frame may be varied for a number of reasons, whereas the duty cycle of colors produced by a color wheel is fixed. Display manufacturers desire multiple duty cycles in order to adjust the white point (which is monitored in real time) and to account for variations due to aging, temperature and so on.
LED technology, however, has lagged behind arc lamp technology in being able to achieve comparable screen lumens. In order to achieve greater brightness from the LEDs, some manufacturers use screens with higher gains; unfortunately, higher gain screens have a narrower viewing angle. A preferable alternative is to illuminate the DMD with multiple primary color LEDs simultaneously to produce overlap colors (referred to as “overlap” colors because they use simultaneously enabled multiple illuminators). For example, using red, green and blue LEDs, cyan (C) can be made from B+G, yellow (Y) can be made from R+G, magenta (M) can be made from B+R and white can be made from R+B+G. Using multiple illuminators simultaneously increases the amount of light reflected by the DMD, thus increasing the brightness of the picture.
While a RGBCMYW system is possible, it is also possible to use less than all of the overlap colors. For illustration purposes, it will be assumed herein that only the Y and C overlap colors are used.
The video data is typically received by control circuit 17 in an RGB format. The portion of the control circuit 17 that generates the overlap colors is shown in FIG. 5. There are two currently used methods for generating the primary and overlap pixel data from RGB data. These methods, Brilliant Color 1 (BC1) and Brilliant Color 2 (BC2), use multiple lookup tables (LUTs) which are accessed using the RGB data. BC1 differs from BC2 in that the overlap colors are treated as functions of R, G and B in BC1, while each color is treated independently in BC2. The information from the lookup tables is used to map data from RGB to RGBYC (or other combination of primary and overlap colors) pixel data by BC control circuitry 42. The mapping data in the LUTs is dependent upon the RGB duty cycle, which heretofore has been fixed.
As explained above, it is desirable to have multiple RGB duty cycles for maintaining a proper color balance during operation of the display and to adjust for the effects of aging. However, multiple duty cycles provide significant hurdles for generating overlap colors to increase brightness. Importantly, if the duty cycles can be switched in real time, the data in the LUTs 40 would need to be replaced for each switch of the RGB duty cycle changed in order to maintain consistent colors. Changing the data in the LUTs would take significant time relative to the operations of the controller 17, thus causing noticeable artifacts. Further, even if duty cycle switches are not made in real time, storage of mapping data for each possible duty cycle would require a significant amount of FLASH memory.
Therefore, a need has arisen for an efficient method and apparatus for maintaining color consistency over multiple RGB duty cycles.